Much of downtown San Diego sits dark Thursday after a blackout struck Southern California and parts of Arizona and Mexico, home to about 6 million people.

Sandy HuffakerGetty Images

Originally published on May 23, 2012 9:26 am

If you thought that the nation's electrical grid was designed to prevent a single, localized malfunction from triggering a blackout for millions of people, you'd be right.

But that didn't prevent that exact event from happening Thursday in San Diego, parts of Arizona, and Mexico's Baja peninsula. Phoenix-based Arizona Public Service Co. said the blackout started when a piece of monitoring equipment was removed at a substation in Yuma, along the border with Mexico.

Andrew Philips, director of transmission and substations at the Electric Power Research Institute, said "any one piece of equipment at any part of the system should not have caused this to happen."

To make matters worse, the power fluctuation caused two reactors at the San Onofre nuclear power station to shut down — cutting off even more of San Diego's power supply.

The Arizona utility declined to speculate on the exact chain of events, pending an investigation.

Mike Niggli, chief operating officer of San Diego Gas & Electric Co., said it's possible extreme heat in the region also may have caused some problems with the transmission lines.

Anjan Bose, a professor of electrical engineering at Washington State University, agrees that if the system was near capacity because of high usage, it could have contributed to the problem.

Bose said the main transmission lines from Arizona to Southern California might have been tripped by whatever happened at the substation, causing the power to reroute to lower-voltage lines that in turn overloaded and tripped.

"Whether it was human error or some malfunction of equipment, we don't know," Bose said. "Usually in these cases it is a bit of both."

Bose said the sort of outage that occurred Thursday is very unusual.

"When someone is fixing something in a substation, they would have to switch off certain things, and there's all sorts of procedures to that," he said. "So, it could have been human error or it could some piece of equipment malfunctioned."

A similar recent problem was caused by human error. In February 2008, about 1 million utility customers in Florida were cut off after a cascading effect with similarities to the Arizona outage. In a report, Florida Power & Light Co. concluded that a field engineer had disabled the systems designed to isolate problems to one piece of equipment or one location.